Gasification is a process that enables the conversion of carbonaceous feedstock, such as municipal solid waste (MSW) or coal, into a combustible gas. The gas can be used to generate electricity, steam or as a basic raw material to produce chemicals and liquid fuels.
Possible uses for the gas include: the combustion in a boiler for the production of steam for internal processing and/or other external purposes, or for the generation of electricity through a steam turbine; the combustion directly in a gas turbine or a gas engine for the production of electricity; fuel cells; the production of methanol and other liquid fuels; as a further feedstock for the production of chemicals such as plastics and fertilizers; the extraction of both hydrogen and carbon monoxide as discrete industrial fuel gases; and other industrial applications.
Generally, the gasification process consists of feeding carbonaceous feedstock into a heated chamber (the gasifier) along with a controlled and/or limited amount of oxygen and optionally steam. In contrast to incineration or combustion, which operate with excess oxygen to produce CO2, H2O, SOx, and NOx, gasification processes produce a raw gas composition comprising CO, H2, H2S, and NH3. After clean-up, the primary gasification products of interest are H2 and CO.
Useful feedstock can include any municipal waste, waste produced by industrial activity and biomedical waste, sewage, sludge, coal, heavy oils, petroleum coke, heavy refinery residuals, refinery wastes, hydrocarbon contaminated soils, biomass, and agricultural wastes, tires, and other hazardous waste. Depending on the origin of the feedstock, the volatiles may include H2O, H2, N2, O2, CO2, CO, CH4, H2S, NH3, C2H6, unsaturated hydrocarbons such as acetylenes, olefins, aromatics, tars, hydrocarbon liquids (oils) and char (carbon black and ash).
As the feedstock is heated, water is the first constituent to evolve. As the temperature of the dry feedstock increases, pyrolysis takes place. During pyrolysis the feedstock is thermally decomposed to release tars, phenols, and light volatile hydrocarbon gases while the feedstock is converted to char.
Char comprises the residual solids consisting of organic and inorganic materials. After pyrolysis, the char has a higher concentration of carbon than the dry feedstock and may serve as a source of activated carbon. In gasifiers operating at a high temperature (>1,200° C.) or in systems with a high temperature zone, inorganic mineral matter is fused or vitrified to form a molten glass-like substance called slag.
Since the slag is in a fused, vitrified state, it is usually found to be non-hazardous and may be disposed of in a landfill as a non-hazardous material, or sold as an ore, road-bed, or other construction material. It is becoming less desirable to dispose of waste material by incineration because of the extreme waste of fuel in the heating process and the further waste of disposing, as a residual waste, material that can be converted into a useful syngas and solid material.
The means of accomplishing a gasification process vary in many ways, but rely on four key engineering factors: the atmosphere (level of oxygen or air or steam content) in the gasifier; the design of the gasifier; the internal and external heating means; and the operating temperature for the process. Factors that affect the quality of the product gas include: feedstock composition, preparation and particle size; gasifier heating rate; residence time; the plant configuration including whether it employs a dry or slurry feed system, the feedstock-reactant flow geometry, the design of the dry ash or slag mineral removal system; whether it uses a direct or indirect heat generation and transfer method; and the syngas cleanup system. Gasification is usually carried out at a temperature in the range of about 650° C. to 1200° C., either under vacuum, at atmospheric pressure or at pressures up to about 100 atmospheres.
There are a number of systems that have been proposed for capturing heat produced by the gasification process and utilizing such heat to generate electricity, generally known as combined cycle systems.
The energy in the product gas coupled with substantial amounts of recoverable sensible heat produced by the process and throughout the gasification system can generally produce sufficient electricity to drive the process, thereby alleviating the expense of local electricity consumption. The amount of electrical power that is required to gasify a ton of a carbonaceous feedstock depends directly upon the chemical composition of the feedstock.
If the gas generated in the gasification process comprises a wide variety of volatiles, such as the kind of gas that tends to be generated in a low temperature gasifier with a “low quality” carbonaceous feedstock, it is generally referred to as off-gas. If the characteristics of the feedstock and the conditions in the gasifier generate a gas in which CO and H2 are the predominant chemical species, the gas is referred to as syngas. Some gasification facilities employ technologies to convert the raw off-gas or the raw syngas to a more refined gas composition prior to cooling and cleaning through a gas quality conditioning system.
Utilizing plasma heating technology to gasify a material is a technology that has been used commercially for many years. Plasma is a high temperature luminous gas that is at least partially ionized, and is made up of gas atoms, gas ions, and electrons. Plasma can be produced with any gas in this manner. This gives excellent control over chemical reactions in the plasma as the gas might be neutral (for example, argon, helium, neon), reductive (for example, hydrogen, methane, ammonia, carbon monoxide), or oxidative (for example, oxygen, carbon dioxide). In the bulk phase, a plasma is electrically neutral.
Some gasification systems employ plasma heat to drive the gasification process at a high temperature and/or to refine the offgas/syngas by converting, reconstituting, or reforming longer chain volatiles and tars into smaller molecules with or without the addition of other inputs or reactants when gaseous molecules come into contact with the plasma heat, they will disassociate into their constituent atoms. Many of these atoms will react with other input molecules to form new molecules, while others may recombine with themselves. As the temperature of the molecules in contact with the plasma heat decreases all atoms fully recombine. As input gases can be controlled stoichiometrically, output gases can be controlled to, for example, produce substantial levels of carbon monoxide and insubstantial levels of carbon dioxide.
The very high temperatures (3000 to 7000° C.) achievable with plasma heating enable a high temperature gasification process where virtually any input feedstock including waste in as-received condition, including liquids, gases, and solids in any form or combination can be accommodated. The plasma technology can be positioned within a primary gasification chamber to make all the reactions happen simultaneously (high temperature gasification), can be positioned within the system to make them happen sequentially (low temperature gasification with high temperature refinement), or some combination thereof.
The gas produced during the gasification of carbonaceous feedstock is usually very hot but may contain small amounts of unwanted compounds and requires further treatment to convert it into a useable product. Once a carbonaceous material is converted to a gaseous state, undesirable substances such as metals, sulfur compounds and ash may be removed from the gas. For example, dry filtration systems and wet scrubbers are often used to remove particulate matter and acid gases from the gas produced during gasification. A number of gasification systems have been developed which include systems to treat the gas produced during the gasification process.
These factors have been taken into account in the design of various different systems which are described, for example, in U.S. Pat. Nos. 6,686,556, 6,630,113, 6,380,507; 6,215,678, 5,666,891, 5,798,497, 5,756,957, and U.S. Patent Application Nos. 2004/0251241, 2002/0144981. There are also a number of patents relating to different technologies for the gasification of coal for the production of synthesis gases for use in various applications, including U.S. Pat. Nos. 4,141,694; 4,181,504; 4,208,191; 4,410,336; 4,472,172; 4,606,799; 5,331,906; 5,486,269, and 6,200,430.
Prior systems and processes have not adequately addressed the problems that must be dealt with on a continuously changing basis. Some of these types of gasification systems describe means for adjusting the process of generating a useful gas from the gasification reaction. Accordingly, it would be a significant advancement in the art to provide a system that can efficiently gasify carbonaceous feedstock in a manner that maximizes the overall efficiency of the process, and/or the steps comprising the overall process.
Gas generated from a gasification reactor may contain heavy metal contaminants such as cadmium, mercury and lead. These heavy metals have emission limits, so before sending gas to downstream applications the heavy metals must be separated from the gas to meet the emission limits for these heavy metals. Examples of emission limits for heavy metals are as follows:
TABLE 1Emission limits for heavy metalsHeavy MetalsEmission LimitsCadmium14 μg/Rm3Lead142 μg/Rm3 Mercury20 μg/Rm3
The composition of the raw gas resulting from coal gasification varies depending on the conditions under which the converter is operated. Common components in the raw gas include combustibles (CO and H2), non-combustibles (CO2, N2 and H2O), air pollutants (heavy metals, NOx, H2S, HCl, tars), and entrained solids. Prior to use of the product gas for combustion, generation of electricity, or other applications, the product gas must be processed or refined in order to produce a gas with desired characteristics for the application. Such processing or refining generally involves the removal of heavy metals and acid gases from the product gas.
When the gas is generated from the conversion of municipal solid waste (MSW) in a gasification system the gas contains metal and metallic compounds in both combustible and non-combustible fractions. Normally, lead concentration in the waste is two orders of magnitude higher than cadmium and mercury. Distribution of heavy metals between various residues depends on MSW composition, physiochemical properties of metals and their metallic compounds and the gasification process operating conditions.
Metal compounds with high vapour pressure (Low boiling point) enter the atmosphere easily after being evaporated and are found mostly in product gas. Toxic heavy metal fumes result from volatilization of metallic constituents followed by the condensation of vapour. Since each load of MSW is different from the previous one, it is almost impossible to know the exact heavy metal concentration in the gasification process. An estimate of the average heavy metal concentration in gas generated from a gasification process is shown below.
TABLE 2Estimate of Heavy Metal Concentration in SyngasHeavy MetalConcentration in SyngasCadmium2.9-3.9 mg/Nm3Lead106-147 mg/Nm3 Mercury1.3-1.7 mg/Nm3
Gas conditioning systems for cleaning gas produced by gasification systems have been described. U.S. Patent Application No. 20040251241 describes the use of conventional gas cleaning technology that can be used to remove acid gases from a mixed gas stream.
U.S. Patent Application No. 20040031450 describes a gasification system that uses an acoustic pressure wave to cause agglomeration of particles contained within the combustion stream for easy removal. In one embodiment, a sulphur capturing agent is injected into the fluid channel for not only removing sulphur from the combustion product stream but for also facilitating particle agglomeration.
U.S. Patent Application No. 20040220285 describes a method and system for gasifying biomass. The resulting synthesis gas is passed through a saturation device and an absorption device, both of which are fed with oil. In this way the synthesis gas is scrubbed with oil and tar is substantially removed therefrom.
U.S. Patent Application No. 20040247509 describes a gas cleaning system for use at high temperatures (between about 1,200° F. to about 300° F.) to remove at least a portion of contaminants such as halides, sulphur, particulates, mercury and others from a syngas. The gas cleaning system may include one or more filter vessels coupled in series for removing halides, particulates, and sulphur from the syngas, and is operated by receiving gas at a first temperature and pressure and dropping the temperature of the syngas as the gas flows through the system. The particles removed by the first filter vessel can be sent to a collection hopper where they can be separated into char particles and sorbent particles. The char particles can be returned to the gasifier and the halide laden sorbent can be disposed of or recycled by adding it to the gas entering the first filter vessel. Return of the char particles to the gasifier will require an additional dedicated inlet for addition of the char particles. The gas cleaning system may be used for applications requiring clean syngas such as fuel cell power generation, IGCC power generation, and chemical synthesis.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.